U.S. patent number 8,427,078 [Application Number 13/064,051] was granted by the patent office on 2013-04-23 for light-emitting element driving device and display device.
This patent grant is currently assigned to Sony Corporation. The grantee listed for this patent is Yasushi Katayama, Takahiro Naito, Tatsuki Nishino. Invention is credited to Yasushi Katayama, Takahiro Naito, Tatsuki Nishino.
United States Patent |
8,427,078 |
Naito , et al. |
April 23, 2013 |
Light-emitting element driving device and display device
Abstract
A light-emitting element driving device includes: a plurality of
light emitters; a power supply; a plurality of current control
transistors; a plurality of constant-current circuits; a voltage
selecting circuit; a control circuit; and a voltage controller.
Inventors: |
Naito; Takahiro (Tokyo,
JP), Katayama; Yasushi (Kanagawa, JP),
Nishino; Tatsuki (Tokyo, JP) |
Applicant: |
Name |
City |
State |
Country |
Type |
Naito; Takahiro
Katayama; Yasushi
Nishino; Tatsuki |
Tokyo
Kanagawa
Tokyo |
N/A
N/A
N/A |
JP
JP
JP |
|
|
Assignee: |
Sony Corporation (Tokyo,
JP)
|
Family
ID: |
44971954 |
Appl.
No.: |
13/064,051 |
Filed: |
March 3, 2011 |
Prior Publication Data
|
|
|
|
Document
Identifier |
Publication Date |
|
US 20110285313 A1 |
Nov 24, 2011 |
|
Foreign Application Priority Data
|
|
|
|
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May 19, 2010 [JP] |
|
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2010-115239 |
|
Current U.S.
Class: |
315/307; 315/294;
345/102 |
Current CPC
Class: |
G09G
3/3406 (20130101); G09G 2330/025 (20130101) |
Current International
Class: |
G09G
3/32 (20060101); H05B 37/02 (20060101) |
Field of
Search: |
;315/307,308,291,312,294,297 ;345/82,102 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Chang; Daniel D
Attorney, Agent or Firm: Rader, Fishman & Grauer
PLLC
Claims
What is claimed is:
1. A light-emitting element driving device comprising: a plurality
of light emitters each including at least one light-emitting
element for emitting light at a luminance level depending on a
current flowing therethrough; a power supply for adjusting an
output voltage depending on a signal supplied to a control terminal
of a switch device and supplying the output voltage to ends of said
light emitters; a plurality of current control transistors
connected between respective other ends of said light emitters and
a reference potential, said current control transistors being
rendered conductive by respective energization signals; a plurality
of constant-current circuits connected respectively in series to
said current control transistors between the other ends of said
light emitters and reference potential; a voltage selecting circuit
for selecting a minimum voltage and a maximum voltage from junction
terminal voltages between said current control transistors and said
constant-current circuits; a control circuit for outputting a
signal having a pulse duration depending on the minimum voltage
selected by said voltage selecting circuit, to the control terminal
of said switch device; and a voltage controller for generating a
control voltage so as to make the maximum voltage selected by said
voltage selecting circuit equal to or smaller than a preset maximum
reference voltage and setting the level of said energization
signals to the level of said control voltage.
2. The light-emitting element driving device according to claim 1,
wherein said control circuit controls the output voltage from said
power supply depending on the junction terminal voltage between the
current control transistor and the constant-current circuit which
are connected to one of said light emitters under a maximum forward
voltage; and said voltage controller controls a junction terminal
voltage other than the junction terminal voltage between the
current control transistor and the constant-current circuit which
are connected to one of said light emitters under a minimum forward
voltage, so as not to be equal to or greater than said preset
maximum reference voltage.
3. The light-emitting element driving device according to claim 1,
wherein said current control transistors include field-effect
transistors, respectively, having respective drains connected to
the corresponding other ends of said light emitters and respective
sources connected to the respective constant-current circuits which
are connected in series to said current control transistors.
4. The light-emitting element driving device according to claim 1,
wherein said voltage controller includes: a control amplifier for
generating said control voltage so as to make the maximum voltage
selected by said voltage selecting circuit equal to or smaller than
said preset maximum reference voltage; and a driver responsive to
said control voltage applied as a drive voltage, for supplying said
energization signals at the level of said control voltage to
respective gates of the corresponding current control
transistors.
5. The light-emitting element driving device according to claim 1,
wherein said power supply includes: a switching power supply
including an inductor or a transformer, a capacitor, and a
switching transistor, for adjusting the output voltage by turning
on and off said switching transistor.
6. A display device comprising: a transmissive display unit; an
illumination unit for illuminating said transmissive display unit
with emitted light, said illumination unit including a plurality of
light emitters each including at least one light-emitting element
for emitting light at a luminance level depending on a current
flowing therethrough; and a light-emitting element driving device
for driving the light-emitting elements of said light emitters,
wherein said light-emitting element driving device includes a power
supply for adjusting an output voltage depending on a signal
supplied to a control terminal of a switch device and supplying the
output voltage to ends of said light emitters, a plurality of
current control transistors connected between respective other ends
of said light emitters and a reference potential, said current
control transistors being rendered conductive by respective
energization signals, a plurality of constant-current circuits
connected respectively in series to said current control
transistors between the other ends of said light emitters and
reference potential, a voltage selecting circuit for selecting a
minimum voltage and a maximum voltage from junction terminal
voltages between said current control transistors and said
constant-current circuits, a control circuit for outputting a
signal having a pulse duration depending on the minimum voltage
selected by said voltage selecting circuit, to the control terminal
of said switch device, and a voltage controller for generating a
control voltage so as to make the maximum voltage selected by said
voltage selecting circuit equal to or smaller than a preset maximum
reference voltage and setting the level of said energization
signals to the level of said control voltage.
7. The display device according to claim 6, wherein said control
circuit controls the output voltage from said power supply
depending on the junction terminal voltage between the current
control transistor and the constant-current circuit which are
connected to one of said light emitters under a maximum forward
voltage; and said voltage controller controls a junction terminal
voltage other than the junction terminal voltage between the
current control transistor and the constant-current circuit which
are connected to one of said light emitters under a minimum forward
voltage, so as not to be equal to or greater than said preset
maximum reference voltage.
8. The display device according to claim 6, wherein said current
control transistors include field-effect transistors, respectively,
having respective drains connected to the corresponding other ends
of said light emitters and respective sources connected to the
respective constant-current circuits which are connected in series
to said current control transistors.
9. The display device according to claim 6, wherein said voltage
controller includes: a control amplifier for generating said
control voltage so as to make the maximum voltage selected by said
voltage selecting circuit equal to or smaller than said preset
maximum reference voltage; and a driver responsive to said control
voltage applied as a drive voltage, for supplying said energization
signals at the level of said control voltage to respective gates of
the corresponding current control transistors.
10. The display device according to claim 6, wherein said power
supply includes: a switching power supply including an inductor or
a transformer, a capacitor, and a switching transistor, for
adjusting the output voltage by turning on and off said switching
transistor.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a light-emitting element driving
device for driving a light-emitting element such as a
light-emitting diode (LED) or the like which emits light at a
luminance level depending on a current flowing therethrough, and a
display device having a non-emission transmissive display unit
which incorporates such a light-emitting element driving
device.
2. Description of the Related Art
The backlight of a liquid crystal display panel employs LEDs as its
light source which have replaced a CCFL (Cold Cathode Fluorescent
Lamp) employing a fluorescent tube.
Particularly, backlights including individual primary LEDs such as
red LEDs, green LEDs, and blue LEDs for producing white light
according to an optical additive color synthesis have been used in
television applications as they can easily achieve good color
balances. In recent years, white LEDs with improved color rendition
have widely been used television applications.
An LED basically has such characteristics that its luminance varies
depending on a current supplied thereto, and has a forward voltage
which differs depending on individual LED variations and
temperatures.
Therefore, when LEDs are used as the backlight of a liquid crystal
display panel, a driving device for those LEDs is required to have
constant-current characteristics in order to achieve a constant
uniform luminance level.
There is known a driving device which adopts a PWM control process
for turning on and off a current flowing through an LED with
certain timing and adjusting a luminance level based on the ratio
of the on and off periods in order to adjust the luminance level
stably in a wide dynamic range.
According to one of schemes for realizing the PWM control process,
a switch element is inserted in series to the LED to turn on and
off the LED with prescribed timing (see Japanese Patent Laid-Open
No. 2001-272938), for example).
There is also known a process for turning on and off switch
elements connected in series to LEDs with energization signals to
control switching transistors of a switching power supply such as a
boosting chopper or the like according to a PWM control
process.
FIG. 1 of the accompanying drawings is a circuit diagram, partly in
block form, of a light-emitting element (LED) driving device
according to the related art.
As shown in FIG. 1, an LED driving device 1 includes a
booster-chopper-type switching power supply 2 and a plurality of
light emitters 3-1 through 3-n (n=2 in FIG. 1) as loads including
LED arrays each including a plurality of series-connected LEDs. It
is assumed that n=2 in the description which follows.
The LED driving device 1 also includes a constant-current
controlling switching transistor 4-1 and a constant-current circuit
5-1 which are connected in series to the light emitter 3-1, and a
constant-current controlling switching transistor 4-2 and a
constant-current circuit 5-2 which are connected in series to the
light emitter 3-2.
The LED driving device 1 further includes switch drivers 6-1, 6-2,
a minimum voltage selecting circuit 7, and a control circuit 8.
The switching power supply 2 includes a constant-voltage source
V21, an inductor L21, a diode D21, an electric storage capacitor
C21, a switching transistor SW21, a current detecting resistive
element R21, and nodes ND21 through ND23.
The inductor L21 has an end connected to the constant-voltage
source V21 which has a voltage VDD and an opposite end connected to
the node ND21. The diode D21 has an anode connected to the node
ND21 and a cathode connected to the node ND22. The capacitor C21
has a terminal (electrode) connected to the node ND22 and another
terminal (electrode) connected to a reference potential VSS, e.g.,
a ground potential.
The node ND22 is connected as a voltage output node of the
switching power supply 2 to respective ends of the light emitters
3-1, 3-2.
The switching transistor SW21 includes an NMOS transistor which is
an n-channel field-effect transistor, for example. The switching
transistor SW21 has a drain connected to the node ND21 and a source
connected to an end of the resistive element R21. The other end of
the resistive element R21 is connected to the reference potential
VSS.
The switching power supply 2 thus constructed operates as follows:
The control circuit 8 supplies a PWM-controlled pulse signal to
turn on and off the switching transistor SW21 to boost the voltage
VDD of the constant-voltage source V21. The switching power supply
2 supplies the boosted voltage VDD as a voltage Vo to the ends of
the light emitters 3-1, 3-2.
Each of the light emitters 3-1, 3-2 includes a series-connected
array of LEDs 31 through 3m.
The LED 31 on an end of the series-connected array of each of the
light emitters 3-1, 3-2 has an anode connected to the voltage
output node ND22 of the switching power supply 2.
The LED 3m on the other end of the series-connected array of the
light emitter 3-1 has a cathode connected to the drain (one
terminal) of the switching transistor 4-1.
The LED 3m on the other end of the series-connected array of the
light emitter 3-2 has a cathode connected to the drain (one
terminal) of the switching transistor 4-2.
Each of the light emitters 3-1, 3-2 is not limited to a plurality
of LEDs, but may include a single LED.
The switching transistor 4-1 has a source (other terminal)
connected to a terminal of the constant-current circuit 5-1, whose
other terminal is connected to the reference potential VSS.
The switching transistor 4-1 remains turned on during the period of
an active high level of a pulsed LED energization signal LO1 that
is supplied via the switch driver 6-1 to the gate of the switching
transistor 4-1.
At this time, a current ILED flows into the light emitter 3-1 which
is supplied with the voltage Vo from the switching power supply 2,
energizing the LEDs 31 through 3m of the light emitter 3-1.
The switching transistor 4-1 remains turned off during the period
of a non-active low level of the pulsed LED energization signal LO.
At this time, no current ILED flows into the light emitter 3-1
which is supplied with the voltage Vo from the switching power
supply 2, de-energizing the LEDs 31 through 3m of the light emitter
3-1.
While the switching transistor 4-1 is being energized, a monitor
voltage Vs1 at a junction node ND1 between the switching transistor
4-1 and the constant-current circuit 5-1 is as follows:
The monitor voltage Vs1 is calculated by subtracting the sum
.SIGMA.Vf (=VF) of forward voltages Vf of all the LEDs 31 through
3m of the light emitter 3-1 from the voltage Vo supplied from the
switching power supply 2.
The monitor voltage Vs1 thus calculated does not take into account
a voltage drop across the switching transistor 4-1.
If the switching transistor 4-1 includes a field-effect transistor
(FET), then the monitor voltage Vs1 at the junction node ND1 is
calculated by subtracting the sum .SIGMA.Vf of the forward voltages
Vf of all the LEDs 31 through 3m of the light emitter 3-1 and a
drain-to-source voltage Vds1 of the FET as the switching transistor
4-1 from the voltage Vo supplied from the switching power supply
2.
The constant-current controlling switching transistor 4-2 has a
source (other terminal) connected to a terminal of the
constant-current circuit 5-2, whose other terminal is connected to
the reference potential VSS.
The switching transistor 4-2 remains turned on during the period of
an active high level of a pulsed LED energization signal LO2 that
is supplied via the switch driver 6-2 to the gate of the switching
transistor 4-2.
At this time, a current ILED flows into the light emitter 3-2 which
is supplied with the voltage Vo from the switching power supply 2,
energizing the LEDs 31 through 3m of the light emitter 3-2.
The switching transistor 4-2 remains turned off during the period
of a non-active low level of the pulsed LED energization signal
LO2. At this time, no current ILED flows into the light emitter 3-2
which is supplied with the voltage Vo from the switching power
supply 2, de-energizing the LEDs 31 through 3m of the light emitter
3-2.
While the switching transistor 4-2 is being energized, a monitor
voltage Vs2 at a junction node ND2 between the switching transistor
4-2 and the constant-current circuit 5-2 is as follows:
The monitor voltage Vs2 is calculated by subtracting the sum
.SIGMA.Vf (=VF) of forward voltages Vf of all the LEDs 31 through
3m of the light emitter 3-2 from the voltage Vo supplied from the
switching power supply 2.
The monitor voltage Vs2 thus calculated does not take into account
a voltage drop across the switching transistor 4-2.
If the switching transistor 4-2 includes a field-effect transistor
(FET), then the monitor voltage Vs2 at the junction node ND2 is
calculated by subtracting the sum .SIGMA.Vf of the forward voltages
Vf of all the LEDs 31 through 3m of the light emitter 3-2 and a
drain-to-source voltage Vds2 of the FET as the switching transistor
4-2 from the voltage Vo supplied from the switching power supply
2.
The minimum voltage selecting circuit 7 selects a minimum voltage
Vsmin from the monitor voltages Vs1, Vs2 at the nodes ND1, ND2
which are calculated by subtracting the voltage drops across the
light emitters 3-1, 3-2 and the switching transistors 4-1, 4-2 from
the voltage Vo, and supplies the selected minimum voltage Vsmin to
the control circuit 8.
The control circuit 8 supplies the gate of the switching transistor
SW21 with a pulse signal having a pulse duration depending on the
minimum voltage Vsmin selected by the minimum voltage selecting
circuit 7.
The switching power supply 2 boosts the voltage VDD of the
constant-voltage source V21 by turning on and off the switching
transistor SW21 with the pulse signal supplied to the gate
thereof.
In this manner, the voltage at the constant-current control
terminal of the light emitter 3-1 or 3-2 under the maximum voltage
VF is controlled at a constant level.
SUMMARY OF THE INVENTION
As described above, since the voltage at the constant-current
control terminal of the light emitter 3-1 or 3-2 under the maximum
voltage VF can be controlled at a constant level, the output
voltage Vo of the switching power supply 2 can be controlled at a
minimum voltage required.
As a result, it is possible to apply a sufficient voltage to the
constant-current circuits 5-1, 5-2 to drive the light emitters 3-1,
3-2, each including an LED array as a load, with a constant
current.
However, if the forward voltage Vf of each LED varies to a value
lower than a standard value, for example, then the voltages applied
to the constant-current circuits 5-1, 5-2 increase, increasing
power consumption of the constant-current circuits 5-1, 5-2 and
reducing electric power efficiency thereof.
Particularly, an increase in the power consumption of the
constant-current circuit 5-1 or 5-2 connected to the light emitter
3-1 or 3-2 which is not under the maximum voltage VF is greatly
responsible for a reduction in the electric power efficiency, and
is also liable to cause the constant-current circuit 5-1 or 5-2 to
generate undue heat.
It is desirable to provide a light-emitting element driving device
and a display device which are capable of reducing a maximum
voltage across a constant-current circuit to minimize a power loss
of the constant-current circuit and reduce heat generated
thereby.
According to an embodiment of the present invention, there is
provided a light-emitting element driving device including a
plurality of light emitters each including at least one
light-emitting element for emitting light at a luminance level
depending on a current flowing therethrough, a power supply for
adjusting an output voltage depending on a signal supplied to a
control terminal of a switch device and supplying the output
voltage to ends of the light emitters, a plurality of current
control transistors connected between respective other ends of the
light emitters and a reference potential, the current control
transistors being rendered conductive by respective energization
signals, a plurality of constant-current circuits connected
respectively in series to the current control transistors between
the other ends of the light emitters and reference potential, a
voltage selecting circuit for selecting a minimum voltage and a
maximum voltage from junction terminal voltages between the current
control transistors and the constant-current circuits, a control
circuit for outputting a signal having a pulse duration depending
on the minimum voltage selected by the voltage selecting circuit,
to the control terminal of the switch device, and a voltage
controller for generating a control voltage so as to make the
maximum voltage selected by the voltage selecting circuit equal to
or smaller than a preset maximum reference voltage and setting the
level of the energization signals to the level of the control
voltage.
According to another embodiment of the present invention, there is
provided a display device including a transmissive display unit, an
illumination unit for illuminating the transmissive display unit
with emitted light, the illumination unit including a plurality of
light emitters each including at least one light-emitting element
for emitting light at a luminance level depending on a current
flowing therethrough, and a light-emitting element driving device
for driving the light-emitting elements of the light emitters. The
light-emitting element driving device includes a power supply for
adjusting an output voltage depending on a signal supplied to a
control terminal of a switch device and supplying the output
voltage to ends of the light emitters, a plurality of current
control transistors connected between respective other ends of the
light emitters and a reference potential, the current control
transistors being rendered conductive by respective energization
signals, a plurality of constant-current circuits connected
respectively in series to the current control transistors between
the other ends of the light emitters and reference potential, a
voltage selecting circuit for selecting a minimum voltage and a
maximum voltage from junction terminal voltages between the current
control transistors and the constant-current circuits, a control
circuit for outputting a signal having a pulse duration depending
on the minimum voltage selected by the voltage selecting circuit,
to the control terminal of the switch device, and a voltage
controller for generating a control voltage so as to make the
maximum voltage selected by the voltage selecting circuit equal to
or smaller than a preset maximum reference voltage and setting the
level of the energization signals to the level of the control
voltage.
According to the present invention, it is possible to reduce the
maximum voltage applied to the constant-current circuits, to reduce
the power losses caused by the constant-current circuits, and hence
to reduce the heat generated thereby.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a circuit diagram, partly in block form, of a
light-emitting element (LED) driving device according to the
related art;
FIG. 2 is a circuit diagram, partly in block form, of a
light-emitting element (LED) driving device according to a first
embodiment of the present invention;
FIG. 3 is a circuit diagram, partly in block form, of the
light-emitting element (LED) driving device according to the first
embodiment of the present invention;
FIG. 4 is a circuit diagram, partly in block form, of a
light-emitting element (LED) driving device according to a second
embodiment of the present invention;
FIG. 5 is a block form of a liquid crystal display device according
to a third embodiment of the present invention; and
FIG. 6 is a perspective view of a transmissive LCD panel.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Preferred embodiments of the present invention will be described
below with reference to the drawings.
The preferred embodiments will be described below according to the
following order.
1. First embodiment (first configuration example of light-emitting
element (LED) driving device)
2. Second embodiment (second configuration example of
light-emitting element (LED) driving device)
3. Third embodiment (display device)
1. First Embodiment
FIG. 2 is a circuit diagram, partly in block form, of a
light-emitting element (LED) driving device according to a first
embodiment of the present invention, and FIG. 3 is a circuit
diagram, partly in block form, of the light-emitting element (LED)
driving device according to the first embodiment of the present
invention.
According to the first embodiment, the LED driving device drives
LEDs as light-emitting elements which are electrooptic elements for
emitting light whose luminance varies depending on a current
flowing therethrough.
As shown in FIGS. 2 and 3, the LED driving device, generally
denoted by 100, includes a booster-chopper-type switching power
supply 110 and a plurality of light emitters 120-1 through 120-n
(n=2 in FIGS. 2 and 3) as loads including LED arrays each including
a plurality of series-connected LEDs. It is assumed that n=2 in the
description which follows.
The LED driving device 100 also includes a constant-current
controlling switching transistor 130-1 and a constant-current
circuit 140-1 which are connected in series to the light emitter
120-1, and a constant-current controlling switching transistor
130-2 and a constant-current circuit 140-2 which are connected in
series to the light emitter 120-2.
The LED driving device 100 further includes switch drivers 150-1,
150-2, a voltage selecting circuit 160, a control circuit 170, a
reference voltage source 180, and a control amplifier 190.
The switch drivers 150-1, 150-2, the reference voltage source 180,
and the control amplifier 190 jointly make up a voltage
controller.
The switching power supply 110 includes a constant-voltage source
V111, an inductor L111, a diode D111, an electric storage capacitor
C111, a switching transistor SW111, a current detecting resistive
element R111, and nodes ND111 through ND113.
The inductor L111 has an end connected to the constant-voltage
source V111 which has a voltage VDD and an opposite end connected
to the node ND111. The diode D111 has an anode connected to the
node ND111 and a cathode connected to the node ND112. The capacitor
C111 has a terminal (electrode) connected to the node ND112 and
another terminal (electrode) connected to a reference potential
VSS, e.g., a ground potential.
The node ND112 is connected as a voltage output node of the
switching power supply 110 to respective ends of the light emitters
120-1, 120-2 as loads.
The switching transistor SW111 includes an NMOS transistor which is
an n-channel field-effect transistor, for example. The switching
transistor SW111 has a drain connected to the node ND111 and a
source connected to an end of the resistive element R111. The other
end of the resistive element R111 is connected to the reference
potential VSS.
The switching power supply 110 thus constructed operates as
follows: The control circuit 170 supplies a PWM-controlled pulse
signal to turn on and off the switching transistor SW111 to boost
the voltage VDD of the constant-voltage source V111 into a boosted
voltage Vo.
The switching power supply 110 supplies the boosted voltage Vo to
the ends of the light emitters 120-1, 120-2.
Each of the light emitters 120-1, 120-2 includes a series-connected
array of LEDs 121 through 12m.
The LEDs 121 on ends of the series-connected arrays of the
respective light emitters 120-1, 120-2 have anodes connected in
common to the voltage output node ND112 of the switching power
supply 110.
The LED 12m on the other end of the series-connected array of the
light emitter 120-1 has a cathode connected to the drain (one
terminal) of the switching transistor 130-1.
The LED 12m on the other end of the series-connected array of the
light emitter 120-2 has a cathode connected to the drain (one
terminal) of the switching transistor 130-2.
Each of the light emitters 120-1, 120-2 is not limited to a
plurality of LEDs, but may include a single LED.
The switching transistor 130-1 has a source (other terminal)
connected to a terminal of the constant-current circuit 140-1,
whose other terminal is connected to the reference potential
VSS.
The switching transistor 130-1 remains turned on during the period
of an active high level of a pulsed LED energization signal LO1
that is supplied via the switch driver 150-1 to the gate of the
switching transistor 130-1.
At this time, a current ILED1 flows into the light emitter 120-1
which is supplied with the voltage Vo from the switching power
supply 110, energizing the LEDs 121 through 12m of the light
emitter 120-1.
The switching transistor 130-1 remains turned off during the period
of a non-active low level of the pulsed LED energization signal LO.
At this time, no current ILED1 flows into the light emitter 120-1
which is supplied with the voltage Vo from the switching power
supply 110, de-energizing the LEDs 121 through 12m of the light
emitter 120-1.
While the switching transistor 130-1 is being energized, a monitor
voltage Vs1 at a junction node ND11 between the switching
transistor 130-1 and the constant-current circuit 140-1 is as
follows:
Basically, the monitor voltage Vs1 is calculated by subtracting the
sum .SIGMA.Vf (=VF) of forward voltages Vf of all the LEDs 121
through 12m of the light emitter 120-1 from the voltage Vo supplied
from the switching power supply 110.
The monitor voltage Vs1 thus calculated does not take into account
a voltage drop across the switching transistor 130-1.
If the switching transistor 130-1 includes a field-effect
transistor (FET), for example, then the monitor voltage Vs1 at the
junction node ND11 is calculated by subtracting the sum .SIGMA.Vf
of the forward voltages Vf of all the LEDs 121 through 12m of the
light emitter 120-1 and a drain-to-source voltage Vds1 of the FET
as the switching transistor 130-1 from the voltage Vo supplied from
the switching power supply 110.
The switching transistor 130-2 has a source (other terminal)
connected to a terminal of the constant-current circuit 140-2,
whose other terminal is connected to the reference potential
VSS.
The switching transistor 130-2 remains turned on during the period
of an active high level of a pulsed LED energization signal LO2
that is supplied via the switch driver 150-2 to the gate of the
switching transistor 130-2.
At this time, a current ILED2 flows into the light emitter 120-2
which is supplied with the voltage Vo from the switching power
supply 110, energizing the LEDs 121 through 12m of the light
emitter 120-2.
The switching transistor 130-2 remains turned off during the period
of a non-active low level of the pulsed LED energization signal LO.
At this time, no current ILED2 flows into the light emitter 120-2
which is supplied with the voltage Vo from the switching power
supply 110, de-energizing the LEDs 121 through 12m of the light
emitter 120-2.
While the switching transistor 130-2 is being energized, a monitor
voltage Vs2 at a junction node ND12 between the switching
transistor 130-2 and the constant-current circuit 140-2 is as
follows:
Basically, the monitor voltage Vs2 is calculated by subtracting the
sum .SIGMA.Vf (=VF) of forward voltages Vf of all the LEDs 121
through 12m of the light emitter 120-2 from the voltage Vo supplied
from the switching power supply 110.
The monitor voltage Vs2 thus calculated does not take into account
a voltage drop across the switching transistor 130-2.
If the switching transistor 130-2 includes a field-effect
transistor (FET), for example, then the monitor voltage Vs2 at the
junction node ND12 is calculated by subtracting the sum .SIGMA.Vf
of the forward voltages Vf of all the LEDs 121 through 12m of the
light emitter 120-2 and a drain-to-source voltage Vds2 of the FET
as the switching transistor 130-2 from the voltage Vo supplied from
the switching power supply 110.
The voltage selecting circuit 160 selects a minimum voltage Vsmin
and a maximum voltage Vsmax from the monitor voltages Vs1, Vs2 at
the nodes ND11, ND12 which are calculated by subtracting the
voltage drops across the light emitters 120-1, 120-2 and the
switching transistors 130-1, 130-2 from the voltage Vo.
The voltage selecting circuit 160 supplies the selected minimum
voltage Vsmin to the control circuit 170, and supplies the selected
maximum voltage Vsmax to the control amplifier 190.
The control circuit 170 supplies the gate of the switching
transistor SW111 with a pulse signal having a pulse duration
depending on the minimum voltage Vsmin selected by the voltage
selecting circuit 160.
The switching power supply 110 boosts the voltage VDD of the
constant-voltage source V111 by turning on and off the switching
transistor SW111 with the pulse signal supplied to the gate
thereof.
In this manner, the voltage at the constant-current control
terminal of the light emitter 120-1 or 120-2 under a maximum
voltage VF is controlled at a constant level.
The control amplifier 190 has an inverting input terminal (-)
supplied with the maximum voltage Vsmax selected by the voltage
selecting circuit 160 and a noninverting terminal (+) supplied with
a preset maximum reference voltage Vrefmax from the reference
voltage source 180.
The control amplifier 190 generates a control voltage Vg for making
the maximum voltage Vsmax equal to the preset maximum reference
voltage Vrefmax, and outputs the control voltage Vg as an operating
voltage for the switch drivers 150-1, 150-2.
When the control voltage Vg is applied as the operating voltage to
the switch drivers 150-1, 150-2, the gate voltages of the switching
transistors 130-1, 130-2 become the voltages at the active high
level of the LED energization signals LO1, LO2.
When the switching transistors 130-1, 130-2 are turned on, the
control voltage Vg is supplied to the gates thereof.
The monitor voltage for the light emitter 120-2 or 120-1 under a
minimum voltage VF is limited so as not to be equal to or higher
than the preset maximum reference voltage Vrefmax.
As Vg.noteq.Vrefmax in this case, the relationship
Vrefmax=Vsmax=Vg-Vgs1 or Vgs2 (a maximum one of the monitor
voltages Vs1, Vs2) is achieved.
Operation of the LED driving device 100 thus constructed will be
described below.
The switching power supply 110 supplies the boosted voltage Vo to
the ends of the light emitters 120-1, 120-2 as loads.
The switching transistor 130-1 that is connected to the light
emitter 120-1 remains turned on during the period of the active
high level of the pulsed LED energization signal LO1 that is
supplied via the switch driver 150-1 to the gate of the switching
transistor 130-1.
At this time, the current ILED1 flows into the light emitter 120-1
which is supplied with the voltage Vo from the switching power
supply 110, energizing the LEDs 121 through 12m of the light
emitter 120-1.
The switching transistor 130-1 remains turned off during the period
of the non-active low level of the pulsed LED energization signal
LO. At this time, no current ILED flows into the light emitter
120-1 which is supplied with the voltage Vo from the switching
power supply 110, de-energizing the LEDs 121 through 12m of the
light emitter 120-1.
While the switching transistor 130-1 is being energized, the
monitor voltage Vs1 at the junction node ND11 between the switching
transistor 130-1 and the constant-current circuit 140-1 is supplied
to the voltage selecting circuit 160.
The switching transistor 130-2 that is connected to the light
emitter 120-2 remains turned on during the period of the active
high level of the pulsed LED energization signal LO2 that is
supplied via the switch driver 150-2 to the gate of the switching
transistor 130-2.
At this time, the current ILED2 flows into the light emitter 120-2
which is supplied with the voltage Vo from the switching power
supply 110, energizing the LEDs 121 through 12m of the light
emitter 120-2.
The switching transistor 130-2 remains turned off during the period
of the non-active low level of the pulsed LED energization signal
LO2. At this time, no current ILED2 flows into the light emitter
120-2 which is supplied with the voltage Vo from the switching
power supply 110, de-energizing the LEDs 121 through 12m of the
light emitter 120-2.
While the switching transistor 130-2 is being energized, the
monitor voltage Vs2 at the junction node ND12 between the switching
transistor 130-2 and the constant-current circuit 140-2 is supplied
to the voltage selecting circuit 160.
The voltage selecting circuit 160 selects the minimum voltage Vsmin
and the maximum voltage Vsmax from the monitor voltages Vs1, Vs2 at
the nodes ND11, ND12 which are calculated by subtracting the
voltage drops across the light emitters 120-1, 120-2 and the
switching transistors 130-1, 130-2 from the voltage Vo.
The voltage selecting circuit 160 supplies the selected minimum
voltage Vsmin to the control circuit 170, and supplies the selected
maximum voltage Vsmax to the control amplifier 190.
The control circuit 170 generates a pulse signal having a pulse
duration depending on the difference between the minimum voltage
Vsmin selected by the voltage selecting circuit 160 and a voltage
VN113 at the node ND113, and supplies the generated pulsed signal
to the gate of the switching transistor SW111 of the switching
power supply 110.
The switching power supply 110 boosts the voltage VDD of the
constant-voltage source V111 by turning on and off the switching
transistor SW111 with the pulse signal supplied to the gate
thereof.
In this manner, the voltage at the constant-current control
terminal of the light emitter 120-1 or 120-2 under the maximum
voltage VF is controlled at a constant level.
The control amplifier 190 generates the control voltage Vg for
making the maximum voltage Vsmax equal to the preset maximum
reference voltage Vrefmax, and outputs the control voltage Vg as an
operating voltage for the switch drivers 150-1, 150-2.
When the control voltage Vg is applied as the operating voltage to
the switch drivers 150-1, 150-2, the gate voltages of the switching
transistors 130-1, 130-2 become the voltages at the active high
level of the LED energization signals LO1, LO2.
When the switching transistors 130-1, 130-2 are turned on, the
control voltage Vg is supplied to the gates thereof.
The monitor voltage for the light emitter 120-2 or 120-1 under the
minimum voltage VF is limited so as not to be equal to or higher
than the preset maximum reference voltage Vrefmax.
A power loss caused by the LED driving device 100 according to the
embodiments of the present invention will be described below in
comparison with the LED driving device shown in FIG. 1 which will
be referred to as a comparative example.
[Power Loss Caused by the LED Driving Device 1 According to the
Comparative Example Shown in FIG. 1]
First, a power loss caused by the LED driving device 1 according to
the comparative example shown in FIG. 1 will be described
below.
It is assumed as preconditions that the currents ILED flowing
through the light emitters 3-1, 3-2 are of 500 mA, the overall
forward voltage VF1 of the light emitter 3-1 is of 50 V, and the
overall forward voltage VF2 of the light emitter 3-2 is of 45
V.
It is also assumed as preconditions that the control voltage for
the minimum voltage Vsmin is of 0.5 V, and the switching
transistors 4-1, 4-2 have respective gate-to-source voltages Vgs1,
Vgs2 which are equal to each other (Vgs1=Vgs2).
It is further assumed as preconditions that the switching
transistors 4-1, 4-2 have respective on resistances Ro (N1), Ro
(N2) of 1 ohms (.OMEGA.) (Ro (N1)=Ro (N2)=.OMEGA.) and respective
drain-to-source voltages Vds1, Vds2, and the constant-current
circuits 5-1, 5-2 have respective voltages VILED1, VILED2 developed
thereacross.
Based on the above preconditions, the output voltage Vo of the
switching power supply 110 is calculated as follows:
.times..times..times..times..times..times..times..times..times..times..ti-
mes..times..times..OMEGA..times..times..times..times.
##EQU00001##
The voltage VILED2 across the constant-current circuit is
calculated as follows:
.times..times..times..times..times..times..times..times..times..times..ti-
mes..times..times..times..times..OMEGA..times..times.
##EQU00002##
A power loss Pd (ILED2) caused by the constant-current circuit 5-2
associated with the light emitter 3-2 which has the lower forward
voltage VF and a power loss Pd (N2) caused by the switching
transistor 4-2 are calculated as follows: Pd(ILED2)=5.5 V.times.100
mA=550 mW Pd(N2)=(100 mA).sup.2.times.1.OMEGA.=10 mW
A power loss Pd (ILED1) caused by the constant-current circuit 5-1
associated with the light emitter 3-1 which has the higher forward
voltage VF and a power loss Pd (N1) caused by the switching
transistor 4-1 are calculated as follows: Pd(ILED1)=0.5 V.times.100
mA=50 mW Pd(N1)=(100 mA).sup.2.times.1.OMEGA.=10 mW
With the LED driving device 1 according to the comparative example
shown in FIG. 1, therefore, the power loss Pd (ILED2) caused by the
constant-current circuit 5-2 associated with the light emitter 3-2
which has the lower forward voltage VF is of 550 mW, and the power
loss Pd (ILED1) caused by the constant-current circuit 5-1
associated with the light emitter 3-1 which has the higher forward
voltage VF is of 50 mW.
Consequently, the constant-current circuit 5-2 associated with the
light emitter 3-2 which has the lower forward voltage VF causes a
much larger power loss (the power loss Pd (ILED2)) that is eleven
times the power loss Pd (ILED2) caused by the constant-current
circuit 5-2.
The power loss Pd (N2) caused by the switching transistor 4-2 and
the power loss Pd (N1) caused by the switching transistor 4-1 are
of 10 mW and equal to each other.
[Power Loss Caused by the LED Driving Device 100 According to the
Present Embodiment]
Next, a power loss caused by the LED driving device 100 according
to the present embodiment will be described below.
For an easier understanding of the present embodiment, the various
parameters are denoted by reference characters which are identical
to those of the comparative example.
It is assumed as preconditions that the current ILED1, ILED2
flowing through the light emitters 120-1, 120-2 are of 500 mA, the
overall forward voltage VF1 of the light emitter 120-1 is of 50 V,
and the overall forward voltage VF2 of the light emitter 120-2 is
of 45 V.
It is also assumed as preconditions that the control voltage for
the minimum voltage Vsmin is of 0.5 V, the control voltage for the
maximum voltage Vsmax is of 1.0 V, and the switching transistors
130-1, 130-2 have respective gate-to-source voltages Vgs1, Vgs2
which are equal to each other (Vgs1=Vgs2).
It is further assumed as preconditions that the switching
transistors 130-1, 130-2 have respective on resistances Ro (N1), Ro
(N2) of 1 ohms (Q) (Ro (N1)=Ro (N2)=0) and respective
drain-to-source voltages Vds1, Vds2, and the constant-current
circuits 140-1, 140-2 have respective voltages VILED1, VILED2
developed thereacross.
Based on the above preconditions, the output voltage Vo of the
switching power supply 110 is calculated as follows:
.times..times..times..times..times..times..times..times..times..times..ti-
mes..times..times..OMEGA..times..times..times..times.
##EQU00003##
The voltage VILED2 across the constant-current circuit 140-2 and
the drain-to-source voltage Vds2 of the switching transistor 130-2
are calculated as follows:
.times..times..times..times..times..times..times..times.
##EQU00004##
.times..times..times..times..times..times..times..times..times..times..ti-
mes..times..times..times..times. ##EQU00004.2##
A power loss Pd (ILED2) caused by the constant-current circuit
140-2 associated with the light emitter 120-2 which has the lower
forward voltage VF and a power loss Pd (N2) caused by the switching
transistor 130-2 are calculated as follows: Pd(ILED2)=1.0
V.times.100 mA=100 mW Pd(N2)=4.6 V.times.100 mA=460 mW
A power loss Pd (ILED1) caused by the constant-current circuit
140-1 associated with the light emitter 120-1 which has the higher
forward voltage VF and a power loss Pd (N1) caused by the switching
transistor 130-1 are calculated as follows: Pd(ILED1)=0.5
V.times.100 mA=50 mW Pd(N1)=(100 mA).sup.2.times.1.OMEGA.=10 mW
With the LED driving device 100 according to the present
embodiment, therefore, the power loss Pd (ILED2) caused by the
constant-current circuit 140-2 associated with the light emitter
120-2 which has the lower forward voltage VF is of 100 mW, and the
power loss Pd (N2) caused by the switching transistor 130-2 is of
460 mW.
Consequently, the LED driving device 100 according to the present
embodiment makes it possible to assign desired power losses to the
constant-current (ILED) circuit and the switch (NMOS) by setting
the control voltage for the maximum voltage Vmax.
According to the present embodiment, the LED driving device 100
allows heat sources to be distributed and can be designed for heat
optimization with increased ease.
If the LED driving device 100 has a plurality of constant-current
(ILED) circuits such as four channels, eight channels, and so on,
then it is effective to be able to set upper limits for power loss
as desired from the standpoint of allowable losses (heat).
The power loss Pd (ILED1) caused by the constant-current circuit
140-1 associated with the light emitter 120-1 which has the higher
forward voltage VF is of 50 mW, and the power loss Pd (N1) caused
by the switching transistor 130-1 is of 10 mW.
As described above, the first embodiment of the present invention
offers the following advantages:
If the forward voltage Vf of each LED varies to a value lower than
a standard value, for example, then the LED driving device 100
according to the present embodiment operates as follows:
The voltages applied to the constant-current circuits can be
reduced, reducing power consumption of the constant-current
circuits and preventing electric power efficiency thereof from
being reduced.
Particularly, it is possible to assign desired power consumption
rates to the constant-current circuit 140-2 or 140-1 associated
with the light emitter 120-2 or 120-1 which is not under the
maximum voltage VF.
As a result, the power losses caused by the constant-current
circuits are reduced and hence the heat generated thereby is
reduced.
2. Second Embodiment
FIG. 4 is a circuit diagram, partly in block form, of a
light-emitting element (LED) driving device 100A according to a
second embodiment of the present invention.
The LED driving device 100A according to the second embodiment is
different from the LED driving device 100 according to the first
embodiment as follows:
The power supply 110 of the LED driving device 100 according to the
first embodiment includes a booster-chopper-type switching power
supply.
However, the LED driving device 100A according to the second
embodiment has a power supply 110A which includes a current-mode
flyback converter including a transformer TRS111.
The other details of the LED driving device 100A according to the
second embodiment are identical to those of the LED driving device
100 according to the first embodiment.
The LED driving device 100A according to the second embodiment
offers the same advantages as those of the LED driving device 100
according to the first embodiment.
The LED driving devices 100, 100A according to the first and second
embodiments are suitable for use in transmissive liquid crystal
display devices which incorporate a backlight device.
3. Third Embodiment
A liquid crystal display device according to a third embodiment of
the present invention, which incorporates an LED backlight to which
the LED driving devices 100, 100A according to the first and second
embodiments shown in FIGS. 2 through 4 are applicable, will be
described below.
FIG. 5 is a block form of a liquid crystal display device 200
according to the third embodiment of the present invention.
As shown in FIG. 5, the liquid crystal display device 200 includes
a transmissive liquid crystal display (LCD) panel 210, a backlight
device 220 as an illumination unit disposed behind the LCD panel
210, an LED driving device 230, and a liquid crystal driver (panel
driving circuit) 240.
The liquid crystal display device 200 also includes a signal
processor 250, a tuner 260, a controller 270, an audio unit 280
including a speaker 281, and a power supply 290.
FIG. 6 is a perspective view of the transmissive LCD panel 210.
As shown in FIG. 6, the transmissive LCD panel 210 includes a TFT
substrate 211, a counter electrode substrate 212 disposed in
confronting relation to the TFT substrate 211, and a liquid crystal
layer 113 with a twisted nematic (TN) liquid crystal sealed
therein, interposed between the TFT substrate 211 and the counter
electrode substrate 212.
The TFT substrate 211 has a matrix of signal lines 214 and scanning
lines 215, thin-film transistors 216 disposed as switching elements
at the intersections of the signal lines 214 and the scanning lines
215, and pixel electrodes 217 disposed at the intersections of the
signal lines 214 and the scanning lines 215, all disposed on an
inner surface thereof.
The thin-film transistors 216 are successively selected by the
scanning lines 215, and write image signals supplied from the
signal lines 214 into the corresponding pixel electrodes 217. The
counter electrode substrate 212 has counter electrodes 218 and
color filters 219 disposed on an inner surface thereof.
The transmissive LCD panel 210 is sandwiched between two
polarizers. While the transmissive LCD panel 210 is being
illuminated with white light from the backlight device 220 disposed
therebehind, the transmissive LCD panel 210 is driven in an active
matrix mode to display desired full-color images.
The backlight device 220 includes a light source 221 and a
wavelength selection filter 222.
The light source 221 includes a plurality of LED arrays each
including the light emitter 120 to be driven according to the first
or second embodiment of the present invention.
The backlight device 220 applies light emitted from the light
source 221 through the wavelength selection filter 222 to the
transmissive LCD panel 210 from behind.
The backlight device 220 shown in FIG. 6 is disposed behind the
transmissive LCD panel 210 and includes a direct-lighting backlight
device disposed behind the transmissive LCD panel 210, for
illuminating the transmissive LCD panel 210 from behind.
The light source 221 of the backlight device 220 employs a
plurality of series-connected LEDs as light emission sources.
Specifically, each of the LED arrays includes a plurality of
series-connected LEDs arranged in the plane of the backlight device
220, and the LED arrays are also arranged in the plane of the
backlight device 220.
The backlight device 220 thus constructed is driven by the LED
driving device 230.
The LED driving device 230 may be either one of the LED driving
devices 100, 100A according to the first and second embodiments
shown in FIGS. 2 through 4.
In FIG. 6, the entire light source 221 is illustrated as being
driven by the LED driving device 230. However, each of the LED
arrays of series-connected LEDs may be associated with and driven
by an individual LED driving device.
The liquid crystal driver 240 includes an X driver circuit, a Y
driver circuit, etc. The signal processor 250 supplies separate R,
G, B signals, for example, to the X driver circuit and the Y driver
circuit to drive the LCD panel 210 for thereby displaying color
images based on the separate R, G, B signals.
The signal processor 250 performs signal processing such as chroma
processing on video signals input from the tuner 260 and an
external source, converts the processed video signals from
composite signals into separate R, G, B signals suitable for
driving the LCD panel 210, and supplies the converted separate R,
G, B signals to the panel driving circuit 240, which drives the LCD
panel 210 to display color images based on the separate R, G, B
signals.
The signal processor 250 also extracts audio signals from the video
signals input thereto and supplies the audio signals to the audio
unit 280, which energizes the speaker 281 to produce sounds based
on the audio signals.
The liquid crystal display device 200 thus constructed incorporates
therein either one of the LED driving devices 100, 100A shown in
FIGS. 2 through 4.
In the liquid crystal display device 200, therefore, the voltages
applied to the constant-current circuits can be reduced, reducing
power consumption of the constant-current circuits and preventing
electric power efficiency thereof from being reduced.
Particularly, it is possible to assign desired power consumption
rates to the constant-current circuit associated with the light
emitter which is not under the maximum voltage VF.
As a result, the power losses caused by the constant-current
circuits are reduced and hence the heat generated thereby is
reduced.
The present application contains subject matter related to that
disclosed in Japanese Priority Patent Application JP 2010-115239
filed in the Japan Patent Office on May 19, 2010, the entire
content of which is hereby incorporated by reference.
Although certain preferred embodiments of the present invention
have been shown and described in detail, it should be understood
that various changes and modifications may be made therein without
departing from the scope of the appended claims.
* * * * *